In http, media types are often used in content negotiation, but those can often only say something about the format of the data, not about the semantics used within the data. This is particularly an issue with data in RDF where the same resource can be described in several different ways using different RDF vocabularies (e. g. DublinCore, foaf or schema,org). This presentation and the accompanying position paper (https://www.w3.org/2016/11/sdsvoc/SDSVoc16_paper_14) presented at SDSVoc in Amsterdam, I suggest a new http header to resolve this problem.

1. An http Header for Metadata
Schema Negotiation
Lars G. Svensson
| 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 20161

2. 2
Why is Profile Negotiation Necessary?
| 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
PhotobyEricKilby(CCBY-SA):https://flic.kr/p/6qrXw3
Photobymario(CCBY):https://flic.kr/p/ceSRwU
The profile you expected The profile you got

3. 3 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
– Profile in Accept-/Content-Type-headers (RFC 7231)
– Link-header with rel=“profile” (RFC 6906)
– Link-header with profile-parameter (RFC 5988)
– Prefer-/Preference-Applied-headers (RFC 7240)
– Accept-Schema-/Schema-headers (yet to be specified)
I have looked at five options

4. 4 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
GET /some/resource HTTP/1.1
Accept: text/turtle;q=0.9;profile=”urn:example:profile-1”,
text/turtle;q=0.7;profile=”urn:example:profile-2”
HTTP/1.1 200 OK
Content-Type: text/turtle; profile=”urn:example:profile-2”
Profile in Accept-/Content-Type-headers
(RFC 7231)

5. 5 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
HEAD /some/other/resource HTTP/1.1
Accept: text/turtle;q=0.9,application/rdf+xml;q=0.5
Link: <http://example.com/profile-1>; rel=”profile”
HTTP/1.1 200 OK
Content-type: text/turtle
Link: <http://example.com/profile-1>; rel=”profile”
Link-header with rel=“profile” (RFC 6906)

6. | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
GET /another/thing HTTP/1.1
Accept: application/xml
Link: <urn:example:profiles:schema-2>; rel=”profile”
HTTP/1.1 200 OK
Link: <http://example.org/another/thing>;
rel=”self”;
type=”application/xml”
profile=”urn:example:profiles:schema-2”
Link-header (RFC 5988 with new parameter
“profile”)
6

7. 7 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
HEAD /this/or/that HTTP/1.1
Accept: application/xml
Prefer: profile=<urn:example:some:schema>
HTTP/1.1 200 OK
Content-Type: application/xml
Preference-Applied: profile=<urn:example:some:schema>
Prefer- and Preference-Applied-headers

8. 8 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
GET /final/resource
Accept: application/rdf+xml; q=0.9, text/turtle; q=0.5
Accept-Schema: <http://example.org/shacl/shape-1>; q=0.9,
<http://example.org/shacl/shape-2>; q=0.5
HTTP/1.1 200 OK
Content-Type: application/rdf+xml
Schema: <http://example.org/shacl/shape-2>
Accept-Schema- and Schema-headers

9. 9 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
Pro Con
Accept/Content-Type
+ Profile
 No header registration
necessary
 Requires registration
of profile parameter
for each content-type
 q-value only available
for each combination
of content-type and
profile
Link: rel=”profile”
(RFC 6906)
 No header, parameter
or relation type
registration necessary
 q-values not
supported
 does not support
multiple combinations
of namespace+profile
Link: profile=”…” (RFC
5988)
 No header registration
necessary
 q-values not
supported
 different use in
request and response
 does not support
multiple combinations
of namespace+profile
in request
Now we can look at the consequences

10. 10 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
Pro Con
Prefer/Preference-
Applied
 No header registration
necessary
 Supports combination
of multiple
namespace+profile-
combinations
 q-values not
supported
 not clear if absence of
“Preference-Applied”
was because server
does not understand
“Prefer” or because it
did not honour the
preference stated
 requires registration
of “profile” preference
Accept-
Schema/Schema
 Supports q-values
 fits nicely into content
negotiation framework
 supports combination
of multiple
namespace+profile-
combinations

 Requires registration
of new http-headers
… and more consequences

11. 11 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
– None of the options is perfect (features vs work)
– In order to perform proper content negotiation, we need q-
values
– In any case we need to figure out, how to deal with multiple
profiles and with combinations of namespaces and schemas
My recommendation is a new header

12. 12 | 12 | An http Header for Metadata Schema Negotiation | Dcember 01, 2016
And now I look forward to the discussion!
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